libgcc helpers __cmpsi2/__ucmpsi2

[Peter Bigot <bigotp at acm dot org>]

I'm updating the machine description for a 16-bit microcontroller, and am
trying to eliminate some hand-coded SImode and DImode insns because in
general gcc generates better code automatically.  So far, I've found no case
where I seem to need to implement cbranchsi4 or cbranchdi4.

However, gcc does generate a reference to __ucmpsi2 when compiling a switch
statement with a default clause and a 32-bit integer as the selection
expression.

The mailing list archives suggest that cmpsi2 was at one point a library
routine, but it is not documented in the internals (at least in the 4.5.x
branch where I'm currently working).  libgcc2 provides a presumably trusted
implementation of __ucmpdi2, though except for a switch statement with a
default clause and a 64-bit selection expression I've been unable to find
any way to generate test code that actually uses it
(gcc.c-torture/execute/cmpdi-1.c doesn't.)  I've never been able to get the
signed version __cmpdi2 to show up.

One past message (http://gcc.gnu.org/ml/gcc/2007-08/msg00225.html) suggests
that a similar evocation of __cmpsi2 might be a bug that arose because there
are "two different sizes of signed integers both of which are larger than
word_mode".  Which is the case in the MSP430.

GCC seems fully capable of reducing SImode and DImode comparisons to HImode
in every other situation.  Is generation of this undocumented helper
function worth treating as a bug in the compiler, or is the documentation
incomplete and back-ends (or libgcc2.c) should be providing an
implementation?  If the former, I'll spend some time trying to isolate it;
if the latter, I won't bother.

Peter